1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus capable of forming images on both faces of recording media according to an inkjet method.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, for example, an inkjet recording apparatus is known that uses a recording head (liquid ejection head or liquid-droplet ejection head) for ejecting droplets of ink.
Such an inkjet-type image forming apparatus may include a sheet conveyance section having a conveyance belt to convey a sheet of recording media and a sheet output section having a pair of output rollers to output the sheet, and be capable of forming (printing) images on both faces (first and second faces) of the sheet.
In a configuration described in JP-2001-063019-A, during printing on a first face of a sheet (simplex printing), the sheet is sandwiched between a pair of output rollers and fed by a conveyance belt and the pair of output rollers in a sheet output direction. When duplex printing is performed on a second face of the sheet, the sheet having an image formed (printed) on its first face is switched back by the pair of output rollers rotatable in both forward and reverse directions. The sheet is guided to a non-opposing surface of the conveyance belt not opposing an image forming section and reversed along a refeed path (reverse transport path), and the image forming section forms an image on a second face of the sheet reversed.
In addition, for example, JP-2009-119745-A proposes an image forming apparatus with a recording head (inkjet recording apparatus) to perform only simplex printing. The image forming apparatus has a conveyance belt and pairs of output rollers separately driven to prevent degradation of image quality by maintaining the accuracy of sheet feeding even if the retaining force of the conveyance belt to retain the sheet thereon decreases.
For a control process shown in, e.g., FIGS. 1 to 7 of JP-2009-119745-A, a sub scanning motor 131 for driving a conveyance belt 31 to convey a sheet 5 is synchronized with an output motor 79 for driving an output conveyance unit 7 to convey the sheet 5 for output, and a drive stop timing of a single driving period (ON time period) of the output motor 79 is delayed from a drive stop timing of a single driving period (ON time period) of the sub scanning motor 131 by a delay time t for intermittent driving. When an entry sensor 331 detects the sheet 5, the delay time t is set to be a time t1. By contrast, when the entry sensor 331 does not detect the sheet 5, the delay time t is set to be a time t2 (t2<t1).
Furthermore, for example, JP-2005-148365-A proposes to perform duplex printing by a pair of output rollers having no switchback function, a conveyance belt rotatable in forward and reverse directions, and a duplex unit.
In the configuration described in JP-2005-148365-A, to align a charging start position to charge an attachment belt of a conveyance device with a contact position at which the sheet transported from the duplex unit contacts the conveyance device, the feed timing at which the sheet is fed front the duplex unit is adjusted according to the arrangement of a charger relative to the duplex unit. Thus, the sheet having an image printed on its first face is reversed by the conveyance device (attachment belt) for duplex printing.
For a conventional type of image forming apparatus like that described in JP-2009-119745-A, the pairs of output rollers only output the sheet and do not switch back the sheet. Such a configuration can reduce the feeding force of the pairs of output rollers (for example, in a case of the pairs of output rollers disposed away from one another, the number of output rollers, the number of spurs, the pressure of spurs, the friction coefficient of output rollers, and the direction of grinding output rollers).
For another conventional type of image forming apparatus (inkjet recording apparatus) like that described in JP-2001-063019-A, the sheet is switched back by only the pair of output rollers. In such a configuration, if the feeding force of the pair of output rollers is set to be large and the feeding force of the sheet conveyance section (conveyance belt) is small, the sheet is strained in each sheet feeding operation (in this case, each time the sheet is intermittently fed by the pair of output rollers and the sheet conveyance section) after the sheet is sandwiched by the pair of output rollers, thus reducing the accuracy of sheet feeding or causing noise when the sheet is strained.
In a case in which the sheet conveyance section is the conveyance belt charged by a charging roller, the conveyance force of the conveyance belt is likely to decrease due to deterioration caused by environmental conditions or elapse of time, or dirt or deterioration caused by a contact with, e.g., the charging roller. In addition, in an inkjet type of image forming apparatus that conveys a printed sheet without using such a conveyance belt, paired sheet rollers may he disposed immediately downstream from the image forming section or printing section in a sheet transport direction, thus hampering setting of a large conveyance force.